2 Samuel 3:26-28 New English Translation (NET Bible)
26 Then Joab left David and sent messengers after Abner. They brought him back from the well of Sirah. (But David was not aware of it.) 27 When Abner returned to Hebron, Joab took him aside at the gate as if to speak privately with him. Joab then stabbed him in the abdomen and killed him, avenging the shed blood of his brother Asahel.
28 When David later heard about this, he said, “I and my kingdom are forever innocent before the Lord of the shed blood of Abner son of Ner!